This invention concerns magnesia ceramic compositions, and particularly such compositions which are plastically deformable at high temperatures.
The plastic deformation method of shaping is well known. It is used, for example, to form objects from wet clay, as by throwing on a potter's wheel. It is also known to plastically form metals, either at room temperature or at elevated temperatures, for example by extrusion or forging.
The essential rheological feature of material which can be plastically formed is the presence of a yield point. This is a stress below which the material will not permanently deform, but above which it will do so, but without rupturing. In plastic forming, the material is subjected to a stress above its yield point to form it into the desired shape, it will then maintain its new shape under the effect of stresses below the yield point. For example, a shape newly formed from plastic clay resists deformation under the force of gravity.
For many years material technologists have been searching for refractory materials (i.e., materials which can be used for structural elements operating at elevated temperatures, for example in molten metal processing apparatus) which will plastically deform, but which will hold their new shape, particularly at elevated temperatures.
The present invention provides a magnesia (MgO) ceramic composition which can be plastically deformed without rupturing at elevated temperatures.